Implantable stimulation devices generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a microstimulator device of the type disclosed in U.S. Published Patent Applications 2005/0021108, published Jan. 27, 2005; 2005/0057905, published Mar. 17, 2005; and 2004/0059392, published Mar. 25, 2004, which are all incorporated herein by reference in their entireties. However, the present invention also has applicability in other implantable stimulator devices, such as Spinal Cord Stimulation (SCS) devices, an example of which can be found in U.S. Pat. No. 6,553,263, which is incorporated herein by reference in its entirety.
Microstimulator devices typically comprise a small generally-cylindrical housing which carries electrodes for producing a desired electric stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy for a wide variety of conditions and disorders. A “microstimulator” in the context of this application means an implantable stimulator device in which the body or housing of the device is compact (typically on the order of a few millimeters in diameter by several millimeters to a few centimeters in length) and usually includes or carries stimulating electrodes intended to contact the patient's tissue. However, a “microstimulator” may also or instead have electrodes coupled to the body of the device via a lead or leads, as shown in U.S. patent application Ser. No. 09/624,130, filed Jul. 24, 2000.
Some microstimulators in the prior art contain only one cathode electrode. More specifically, in such devices, and referring to FIG. 1, a single anode electrode 14 is provided for sourcing current into a resistance 16, R, i.e., the user's tissue. Typically, a return path for the current is provided by a single cathode 14′, which could comprise another electrode on the device, but which might also comprised a portion of the conductive case for the device. Such a device is referred to herein as a “bi-electrode microstimulator,” given its two electrodes 14 and 14′. As is known, the anode 14 sources or sinks current using a current generator circuit within a programmable Digital-to-Analog Converter, or “DAC” 20. The cathode 14′ could also be connected to a current generator circuit or could simply be tied to a reference potential. An example of a bi-electrode microstimulator device includes the Bion® device made by Advanced Bionics Corporation of Sylmar, Calif.
Bi-electrode microstimulators benefit from simplicity. Because of their small size, the microstimulator can be implanted at a site requiring patient therapy, and without leads to carry the therapeutic current away from the body as mentioned previously. However, such bi-electrode microstimulators lack therapeutic flexibility: once implanted, the single cathode/anode combination will only recruit nerves in their immediate proximity, which generally cannot be changed unless the position of the device is manipulated in a patient's tissue.
To improve therapeutic flexibility, microstimulators having more than two electrodes have been proposed, and such devices are referred to herein as “multi-electrode microstimulators” to differentiate them from bi-electrode microstimulators discussed above. When increasing the number of electrodes in this fashion, the electrodes can be selectively activated once the device is implanted, providing the opportunity to manipulate therapy without having to manipulate the position of the device.
Drawings of an exemplary multi-electrode microstimulator 400 are shown in various views in FIGS. 2A-2C. As shown, the device 400 comprises a body or housing 402 which incorporates the power source (battery) and other circuitry needed for the device to function. On the exterior of the housing 402 are (in this example) eight conductive connectors 404 which are coupled to current generation circuitry in the housing (not shown). In this particular example, and as best shown in FIGS. 2B and 2C, a laminate 410 is positioned over the housing so as to bring the connectors 404 into contact with contact pads 412. The laminate 410 is akin to a printed circuit board and contains conductors 414 which ultimately meet with electrodes 416 designed to directly contact a patient's flesh. Thus, when the housing 402 and laminate 410 are coupled in this manner (FIG. 2C), the result is a multi-electrode microstimulator in which the various electrodes 416 are carried by and along the body of the device. Further details concerning this and other structures for a multi-electrode microstimulator are disclosed in the following references, which are incorporated herein in their entireties: U.S. Patent Publication No. 2004/0015205, published Jan. 22, 2004; U.S. patent application Ser. No. 11/142,154, filed Jun. 1, 2005; and U.S. patent application Ser. No. 11/280,620, filed Nov. 16, 2005. Additionally, a multi-electrode microstimulator need not employ electrodes on the body 402, and instead or in addition could comprise the structure of FIG. 2A with a lead or leads coupling to connectors 404 (not shown).
An issue concerning the design of any implantable stimulator, and especially microstimulators of the sort discussed above, involves the use of decoupling capacitors. One such decoupling capacitor 25, C, is shown in FIG. 1. As is known, decoupling capacitors are useful in implantable stimulator devices for a number of reasons. First, they can assist in charge recovery after the provision of a stimulation pulse, a point which is well known in the art and does not require further elaboration. Second, they provide additional safety by preventing the direct injection of current from the current generator circuit (e.g., inside of DAC 20) to the patient's tissue 16, R.
Examples of the use of decoupling capacitors in the implantable stimulator art are illustrated in FIGS. 3A and 3B. FIG. 3A shows an example of the use of decoupling capacitors 25 in a Spinal Cord Stimulation (SCS) device 30, such as the Precision® SCS device marketed by Advanced Bionics Corporation. As shown, this implantable stimulator comprises a plurality of electrodes 32, E1-En. Ultimately, a lead extension (not shown) can couple to the electrodes to carry the signals generated by an implantable pulse generator (IPG) to an electrode array (not shown) at the end of a lead. As a result, the electrode array can be tunneled into position (e.g., along the patient's spinal cord), while the IPG is implanted generally at a relative distance (e.g., in the patient's buttocks).
Associated with each electrode E1-En is a corresponding decoupling capacitor 25, C1-Cn. In an SCS device 30, the electrodes can be selectively activated, and any activated electrode can be selected as an anode or cathode. Indeed, more than one electrode can be selected as an anode at one time, and more that one electrode can be selected as a cathode at one time.
Thus, assume that electrode E2 is selected to act as an anode while electrode E4 is selected to act as a cathode as shown in FIG. 3A. Because each electrode E1-En is hardwired with a decoupling capacitor C1-Cn, the resulting current path through the two electrodes E2 and E4 includes decoupling capacitors C2 and C4. This assists in charge recovery at both electrodes, and further provides redundant safety: even if one of the two capacitors C2 or C4 were to fail, the other would prevent the direct injection of current into the tissue R.
This approach of SCS device 30—in which a decoupling capacitor is associated with each electrode—is generally non-problematic. In an SCS device 30, because the IPG is not implanted at the site of required therapy and instead is positioned at a less critical portion of the patient (e.g., in the buttocks), the IPG can generally be made larger than can the body of the microstimulators discussed earlier. For instance, the IPG used in the SCS device 30 might be disk-shaped with a diameter of a few centimeters and a thickness of several millimeters. There is generally sufficient room in the IPG to accommodate the relatively large decoupling capacitors, C1-Cn. Thus, many currently marketed SCS devices 30 employ IPGs having 16 electrodes (17 counting the case electrode) and 16 corresponding decoupling capacitors (17 counting the case).
FIG. 3B illustrates another device 50 in which decoupling capacitors have been used in the implantable stimulator art, and specifically illustrates the use of a decoupling capacitor in the bi-electrode Bion® microstimulator device discussed earlier. As noted, bi-electrode microstimulator 50 comprises a single cathode 52 and anode 52′. As can be seen, a single decoupling capacitor C 25 is coupled to the cathode 52, and specifically is coupled between the cathode electrode 52 and the current generation circuitry 20. The anode, by contrast, is merely grounded or tied to a reference potential. Through the use of the decoupling capacitor, C, the same benefits noted earlier—improved safety and charge recovery—are had. (However, because only one decoupling capacitor is provided in the current path there is no redundant safety as provided by the two decoupling capacitors in the SCS device 30 of FIG. 3A).
As noted earlier, the body 55 of a bi-electrode microstimulator device 50 is very small, meaning there is a reduced volume within the body to accommodate multiple relatively-large decoupling capacitors 25. However, because such a device traditionally required the use of only a single decoupling capacitor, space within the body 55 was generally sufficient to accommodate this component.
However, the issue of limited space within the body of a microstimulator becomes very significant when a multi-electrode microstimulator is contemplated. Consider a multi-electrode microstimulator having eight cathodes and one anode (perhaps comprising the device's case). In such an architecture, and pursuant to the conventional wisdom of the prior art as understood by the Applicants, the microstimulator would need to have eight decoupling capacitors, one each hard-wired to each electrode. But as noted above, a microstimulator is intended to be quite small. This conflict either limits the number of electrodes a multi-electrode microstimulator can carry, or increases body size, neither of which is desirable.
Accordingly, the implantable stimulator art, and particularly the microstimulator art, would benefit from the ability to provide multiple electrodes while still providing sufficient capacitive decoupling that uses minimal volume inside the device. Embodiments of such a solution are provided herein.